Due to the specificity of his claims. Also he was probably very reasonably paranoid during the prep phase of this whole fiasco. Coercion is not an unreasonable expectation for someone in his position and he has the technical chops to implement such a failsafe. I cannot imagine that anyone in his situation with a modicum of technical savvy would simply carry around data that one could be forced to decrypt.

I wouldn't hold your breath. sigzero is not a fan of Snowden, as can be deduced from his other posts on this subject.